The design of communications systems has traditionally been based upon the Open Systems Interconnect (OSI) specification proposed by the International Standards Organization (ISO). The OSI Model partitions functions into 7 protocol layers. The definition of the layers is typically somewhat flexible, but the overall framework is a helpful reference in which to discuss communication systems.
In the powerline system under consideration, 4 of the 7 layers are of interest; the application layer; the transport layer; the medium access layer (MAC) and the physical layer. The application layer deals with protocols for the applications running on the individual devices within the network that the communication system is designed to support. These applications may include video applications such as high and standard definition TV, high quality audio, and Internet Protocol applications, among many others. The support of these applications requires that the communication system support high data rates, and protocol mechanisms to specify, monitor and control the quality of service made available to the application data streams.
The transport layer typically consists of the protocols and methods that are responsible for peer-to-peer transport of application data between devices. These functions may include creation of new packets/frames, definition of logical communication mechanisms such as connections or flows, specify packet-oriented or connection-oriented architecture for transport and define quality of service parameters as well as enforcing them, among other functions. The MAC layer supports the transport layer with such things as acknowledgements for reliable packet delivery, in-sequence packet delivery, multiplexing of connections, etc. The physical layer involves the digital signal processing for the digital transfer of packets between devices. For purposes of this discussion, the MAC layer and the physical layer are only important in how they interact with the transport layer.
As mentioned above, the transport layer specifies whether an architecture is a connection-less architecture, or a connection-oriented architecture. The Internet Protocol (IP) is an example of a connection-less architecture. Packets are transmitted from source to destination along one of several possible routes, where there is generally no pre-definition of what the route may be. Asynchronous Transfer Mode (ATM) is an example of a connection-oriented architecture. ATM systems define a logical communication link between a source and destination(s) called permanent virtual circuits (PVC).